Many modern and/or emerging applications require at least one device electrode that has not only high electrical conductivity, but high optical transparency as well. Such applications include, but are not limited to, touch screens (e.g., analog, resistive, improved analog, X/Y matrix, capacitive), flexible displays (e.g., electro-phoretics, electro-luminescence, electrochromatic), rigid displays (e.g., liquid crystal (LCD), plasma (PDP), organic light emitting diode (LED)), solar cells (e.g., silicon (amorphous, protocrystalline, nanocrystalline), cadmium telluride (CdTe), copper indium gallium selenide (CIGS), copper indium selenide (CIS), gallium arsenide (GaAs), light absorbing dyes, quantum dots, organic semiconductors (e.g., polymers, small-molecule compounds)), fiber-optic communications (e.g., electro-optic and opto-electric modulators) and microfluidics (e.g. electrowetting on dielectric (EWOD)). As used herein, a layer of material or a sequence of several layers of different materials is said to be “transparent” when the layer or layers permit at least 50% of the ambient electromagnetic radiation in relevant wavelengths to be transmitted through the layer or layers. Similarly, layers which permit some but less than 50% transmission of ambient electromagnetic radiation in relevant wavelengths are said to be “semi-transparent.”
Currently, the most common transparent electrodes are transparent conducting oxides (TCOs), specifically indium-tin-oxide (ITO) on glass. However, ITO can be an inadequate solution for many of the above-mentioned applications (e.g., due to its relatively brittle nature and correspondingly inferior flexibility and abrasion resistance), and the indium component of ITO is rapidly becoming a scarce commodity. Additionally, ITO deposition usually requires expensive, high-temperature sputtering, which can be incompatible with many device process flows. Hence, more robust and abundant transparent conductor materials are being explored.
Single-walled carbon nanotubes (SWNTs) have attracted a great deal of interest, due to their unique mechanical and electrical properties. Highly conductive SWNT networks having a dc conductivity of at least about 4000 Siemens/cm and methods of fabricating these together with the ink material that is used for the fabrication have been described in the literature. However, although nanotube networks fabricated to date are both conducting and transparent, they have not been able to achieve the right combination of sheet conductance and transparency to be competitive with currently used materials such as indium-tin-oxide (ITO). (L.Hu et al Nano Letters 4, 2513 (2004)) N. P. Armitage, J-C P Gabriel and G. Gruner, “Langmuir-Blodgett Nanotube Films”, J. Appl. Phys. Lett, 95, 6, 3228-3330 (2003)).